A 19-year-old Petaluma woman studying to be a firefighter disappeared after attending a music festival New Year's Eve in South Lake Tahoe, authorities said Thursday.

Alyssa Byrne was last seen by a friend at a bar at the Horizon Casino Resort in Stateline, Nev., on Tuesday just after midnight after she took a shuttle bus from the three-day SnowGlobe Music Festival at Lake Tahoe Community College, authorities said.

Byrne last used her cell phone at 12:03 a.m. Tuesday, but it wasn't immediately clear if it was a phone call or text, said Douglas County sheriff's Sgt. Pat Brooks. He said each room of the hotel was being searched.

"The circumstances of her disappearance are unknown," Brooks said.

Investigators said they were aware of two messages that were apparently posted on Byrne's Twitter account on New Year's Eve. One read, "After tonight you'll never be apart of my life you won't be a memory of 2013 a new year you'll never be apart your nothing to me."

The most recent message was, "Just lost trust in the person I trusted most."

But Undersheriff Paul Howell cautioned, "We've seen those, and there's still a debate as to the actual author of those tweets." Sheriff's deputies were in Petaluma interviewing acquaintances "to try to nail that down," he said.

Byrne's mother, Kim Byrne, 49, said Thursday that she was optimistic that her daughter would be found.

"I've convinced myself that we're going to bring her home today," she said. "My best-case scenario right now is that she's having the time of her life with some friends that she's made and that time just got away from her. I just have to focus on that. Today, hopefully, she's just going to come barreling through that door."

Kim Byrne said her daughter has never gone missing before and that she didn't know what has happened to her. "We just have no idea," she said. "That's the million-dollar question."

She said she is staying positive "because I have no other choice. There is a door that I cannot walk through right now."

The missing woman's father, Kevin Byrne, 44, was in South Lake Tahoe assisting in the search.

Family members and friends passed out fliers Wednesday and law-enforcement officials examined surveillance video, including from cameras at the hotel, Ross Sullivan, a family friend, wrote on a Facebook page dedicated to the search.

"Keep spreading the love, we will find you Alyssa," Sullivan wrote.

Byrne is white, with black hair and blue eyes, 5 feet 3 inches tall and 125 pounds. She has an "infinity" sign on her left wrist and wears glasses. She was last seen wearing a black sweatshirt, black yoga pants and black snow boots.

A reward of up to $1,000 is being offered for information leading to Byrne's whereabouts. Anyone with information is asked to call South Lake Tahoe police at (775) 782-9905.